This invention relates to a heat treating process for the stabilization of carbon in austenitic alloy material whereby the carbon is stabilized in the form of carbides having chromium-sufficient "envelopes".
High nickel alloys such as Inconnel 600 are particularly useful in tubing for nuclear steam generators, where their good heat transfer and corrosion-resistant properties are desired. The high nickel alloys are typically composed of 99 percent nickel-chromiumiron, and less than 0.05 percent carbon. If such tubes are not stabilized they can be susceptible to corrosion attack during use. This is due to a condition known as sensitization, which occurs at temperatures between 800.degree. and 1400.degree. F. (425.degree. and 760.degree. C.).
Sensitization is a microstructural condition wherein chromium carbide particles are formed in the grain boundaries to produce a network of chromium-deficient "envelopes" around each carbide. Envelope chromium content is not high enough to resist acid attack so the tube is susceptible to intergranular corrosion.
The prior art practice for stabilizing tubing to be used in a steam generator can be summarized as follows. The steam generator fabricator receives finally sized tubes from the tube mill that have been desensitized by the solution anneal method. These tubes have been desensitized by annealing at temperatures high enough to dissolve all chromium carbides. Carbon is thus put in solid solution as an interstitial element, and kept there by rapid cooling. Solution annealed tubing has therefore been desensitized but not stabilized. Subsequent heating into the sensitization temperature range during steam generator fabrication or tube stress relieving will again produce sensitization and susceptibility to corrosion, if the tubes are not stabilized. The prior art method of stabilizing the carbon is to heat the solution annealed tubing for 15 to 21 hours at about 1300.degree. F. (700.degree. C.), long enough for all the carbon to be tied up in the form of chromium-sufficient carbides. The prolonged heating first causes sensitization, i.e., the carbon comes out of solution to form carbides in the grain boundaries. During the latter portion of the prolonged heating, chromium diffuses slowly into the carbide envelopes and thus the tube becomes desensitized and homogenized with all carbon tied up as carbon tied up as carbides in the grain boundaries.
Although stabilization is achieved with the prior art techniques, it would be desirable to stabilize tubes while they are in the intermediate size (about 25 feet or 7.5 m) so that conventional furnaces can be used, and to reduce the heat treatment time so that a continuous stabilization in conveyor furnace can be ahieved.